Laser guide construction

ABSTRACT

A laser construction utilizes a gas which assumes an ionic state at the discharge temperature and in the active discharge region the plasma is contained and &#39;&#39;&#39;&#39;guided&#39;&#39;&#39;&#39; by a tube exhibiting anisotropic and semi-conductor properties, pyrolytic carbon being used as an example. The tube utilizes the thermal and electrical characteristics of the material such that radially of the axis of discharge the tube exhibits high thermal conductivity and axially of the discharge the tube exhibits low thermal conductivity and a semi-conductor electrical character. The tube is held at an anodic potential which enables employment of a unique starting technique and reduction of ionic bombardment between the plasma and the bore of the tube. Other electrical and physical configurations for reducing ion bombardment or &#39;&#39;&#39;&#39;sputtering&#39;&#39;&#39;&#39; are disclosed.

United States Patent McMahan [54] LASER GUIDE CONSTRUCTION [72]inventor: William H. McMahan, Winter Park, Fla. [73] Assignee:ControlLaser-Orlando, Inc., Orlando, Fla.

[ Notice: The portion of the term of this patent subsequent to Dec. 1,1987, has been disclaimed.

[22] Filed: March 9, 1970 [21] Appl. No.: 17,461

Related US. Application Data [62] Division of Ser. No. 655,652, July 24,1967, Pat. No.

3,544,915 12/1970 McMahan ..33l/94.5

[151 3,670,256 [451 *June 13, 1972 Primary Examiner-William L. SikesAttorney-B. B. Olive [57] ABSTRACT A laser construction utilizes a gaswhich assumes an ionic state at the discharge temperature and in theactive discharge region the plasma is contained and guided" by a tubeexhibiting anisotropic and semi-conductor properties, pyrolytic carbonbeing used as an example. The tube utilizes the thermal and electricalcharacteristics of the material such that radially of the axis ofdischarge the tube exhibits high thermal conductivity and axially of thedischarge the tube exhibits low thermal conductivity and asemi-conductor electrical character. The tube is held at an anodicpotential which enables employment of a unique starting technique andreduction of ionic bombardment between the plasma and the bore of thetube. Other electrical and physical configurations for reducing ionbombardment or sputtering are disclosed.

8 Claims, 8 Drawing Figures PATENTEDJuu 13 m2 SHEET 10F 4 INVENTOR.William H. McMahan ATTORNEY PATENTEDJUN 1 3 I972 SHEET 2 OF 4 3 *0 ucsuuocm 339 U QcmmE ummubsu 8 wimm H McMah an ATTOR NEY PATENTEDJUN 13I872 SHEET 30F 4 ATTORNEY PATENTEDJUN 13 I972 SHEET H1!" 4 wmiar'n H.McMahan ATTORNE Y .treated as an oscillator" LASER GUIDE CONSTRUCTIONCROSS REFERENCE TO RELATED APPLICATION This application is a division ofcopending application, Ser. No. 655,652, filed July 24, 1967, titled GasLaser Plasma Guide now issued as US. Pat'. No. 3,544,915.

BACKGROUND OF THE INVENTION 1. Field of the Invention The inventionrelates broadly to devices for generating directional and coherent lightbeams and in current literature such devices are identified as lasers."More specifically the invention is directed to what will be referred toas a laser and to the type of laser which utilizes a gas that assumes anionic state at the discharge temperature. With respect to such a laserthe invention is furthermore concerned with the physical construction ofthe laser and particularly with what is sometimes referred to as thelaser tube which contains and guides the plasma during discharge anddefines the region in which light generation is achieved. The inventionwhen also may be said to be related to molecular or particle typeresonators and to optical amplifiers.

2. Description of the Prior Art Gas lasers are old, an example being thewell known argon ion laser. It is also known to employ various conductorand insulator materials for the laser tube in the active region andamong the most widely used of such prior art materials has been quartz.There has also been produced a gas laser in which in the active regionthere is employed what is effectively a slotted tubular configurationmade up from pressed, porous, amorphous, annular graphite rings whichare electrically out of contact with each other, are spaced from oneanother and which exhibit. individually in the axial direction of thetube an electrical conductivity which is undesirably high. Variousceramic materials, insulated metal sections and tubes of insulatormaterial have been used. In general, it can be said as to prior artmaterials which have been employed for laser tubes that such materialshave either been insulated conductors or insulators and have exhibitedundesirable characteristics both thermally and electrically and this hasheld back the development of the gas laser because of heat problems andelectrical conductivity problems as well as ionic bombardment problemsassociated with starting and maintaining the laser beam. Laser guidematerials have frequently been mounted on or supported by othermaterials for structural purposes and this has led to complex physicalstructures. The prior art has also been directed to amorphous materials,i.e., materials whose electrical and thermal characteristics are notoriented along any particular plane or axis. The life and power outputsobtained have been limited by the choice of materials employed in thelaser tube and by the manner of operation.

In recent years the development of extremely high temperature problemsin space craft and other advanced applications has led to thedevelopment of a new material referred to as pyrolytic" graphite andsuch material has been used for the manufacture of fastening devicessuch as nuts, bolts and screws which have to withstand extremely hightemperatures. Such material is obtainable from the Space Age MaterialsCorporation, 25-26 50th Street, Woodside Long Island, N. Y. ll377. Thismaterial is known to be of an anisotropic nature and when made in theform of a sheet, which is generally the only form in which it has beenpractical to manufacture, exhibits the unusual character of havingextremely low electrical and thermal conductivity in the axialdirection, i.e., the Z axis," perpendicular to the plane of the sheet.Further, in the plane of the sheet, i.e., the basal plane, radially ofthe axis perpendicular to the plane of the sheet, the material exhibitsa relatively high electrical and thermal conductivity. Primary interestin the material has focused on use in space crafi, high temperatureapplications. The possibility of converting such sheet form to a tubularform suitable to laser application and to utilizing the thermal as wellas the electrical properties of such material for a laser tube has notbeen previously recognized or pursued by those working in the laser art.

Prior art articles useful to an understanding of the invention anddealing with gas lasers, laser tube constructions, ionic bombardment,starting and operating techniques and with graphite product propertiesmay be found in the following publications:

1. Applied Physics Letters," Volume 4, Number 10, May

15, 1964, and Volume 7, Number 7, Oct. 1, 1965;

2. IEEE Journal of Quantum Electronics," Volume QE-3, Number 2;February, 1967, and Volume 012-], Number 6; September, 1965.

3. Electrochemical Technology,

An object of the present invention, therefore, is to utilize ananisotropic material for the construction of a laser tube.

Another object is that of utilizing the properties of pyrolytic graphitematerial specifically in the construction of a laser tube.

Another object is to provide a laser tube constructed of a material andin such an electrical and physical configuration that the entire tubecan be started and operated with the tube itself acting as the anode.

Another object is that of providing a laser tube which in a directionradial of the axis of the tube, i.e., the basal plane, exhibits a highthermal conductivity for heat dissipated in the active region of thelaser.

Another object is that of providing a method of constructingself-supporting, electrically and physically integral, tubularconfigurations from a sheet of an anisotropic material.

Another object is that of providing a gas laser incorporating a lasertube which exhibits a semi-conductor electrical property in the axialdirection, i.e., the Z axis of the material, such that when the laser isin a laser mode and the tube is held at an anodic potential, thedischarge is maintained in the gaseous path defined by the bore of thetube and in a manner to minimize the efiect of ion bombardment withinthe bore.

The foregoing and other objects will become apparent as the descriptionproceeds.

SUMMARY OF THE INVENTION According to the invention a laser is providedwhich is generally of the ion and gas laser type and is generallyadapted to utilize any gas which assumes an ionic state at thetemperatures encountered during laser discharge, hydrogen being anexception. Within the active discharge region, the gas is confined in atube formed of an anisotropic material. The tube exhibits asemi-conductor electrical character in the axial or Z axis direction andrelatively high thennal conductivity in the basal plane, i.e., in adirection radial to the axis of the tube, both of which characteristicsare utilized by the invention and particularly in this axial and radialorientation. In the embodiment of the invention disclosed, theanisotropic tube is made up of uniform cylinders which are individuallyformed from a sheet of anisotropic material, namely, pyrolytic graphiteand which are assembled together in a self-supporting tubular form andwhich operates as if it were molded as an integral structure bothelectrically and physically. The cylinders are cut from the sheet withan anisotropic orientation related to the anisotropic orientation of thesheet such that the tube exhibits the desired radial thermalconductivity and axial electrical semi-conductivity and such method ofachieving such an anisotropic tube forms part of the invention.

The laser of the invention may be either of the air cooled or watercooled type, both being hereinafier discussed, and in addition to beingdirected to a laser tube constructed of anisotropic material and to themethod of making such a tube from anison-opic material in sheet form,the invention is further directed to the mode of sustaining the plasmadischarge in a semi-conductor laser tube such that ion bombardment isminimized or eliminated entirely and the life of the tube is therebyconsiderably lengthened. In regard to this aspect of the invention, theinvention relies on employment of a potential on the anode end of thetube after discharge has been obtained and the laser is in a laser modesuch that the issue of May-June,

tube potential, at its anode end, is held at an anodic potential equalto or positive with respect to the potential of the plasma at the anodeend and the potential on the cathode end of the tube is held equal,i.e., neutral or positive, with respect to the plasma potential at thecathode end such that the normal tendency of the gas ions to leave thegas and impinge on the inside surface of the bore of the tube isprevented. Other means for reducing ion bombardment are disclosed andare directed to modifying the shape of the tube or altering the magneticfield in the active region.

Generally, the invention can be said to be directed to the method ofmaking of a self-supporting anisotropic, semi-conductor, laser tube, tothe embodying of such a tube in a gas laser and to the circuitarrangement employed to initiate and maintain the plasma discharge inthe tube once it has been started in a manner to prevent or minimizetube deterioration due to ion bombardment.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective flow diagramillustrating the manner of obtaining an anisotropic tube from a sheet ofanisotropic material;

FIG. 2 is a perspective of a laser system based on air cooled operationand utilizing a laser tube according to the invention;

FIG. 3 is a section view taken generally along line 3-3 of FIG. 2;

FIG. 4 is a perspective view of a laser system based on water cooledoperation and utilizing a laser tube according to the invention;

FIG. 5 is a section view taken generally along line 5-5 of FIG. 4;

FIG. 6 is a longitudinal section view of a tube construction modified toequalize the potential drop along the plasma and tube;

FIG. 7 is a longitudinal section view of a tube constructionillustrating an alternate modified tube for equalizing the potentialdrop along the plasma and tube; and

FIG. 8 represents schematically an arrangement for equalizing tube andplasma potentials by adjusting the magnetic field.

DESCRIPTION OF THE PREFERRED EMBODIMENTS As has heretofore beenmentioned much of the invention is directed to the employment of a lasertube formed of an anisotropic material. While it is believed that manymaterials will eventually prove to have a suitable semi-conductorelectrical characterto enjoy certain aspects of the inventionparticularly those aspects related to ion bombardment reduction, amaterial known to be peculiarly adapted to the invention is thepreviously identified pyrolytic graphite. This material is presentlyknown to be available only in sheet form as represented by sheet 10 ofFIG. 1 and in this form the sheet exhibits relatively high electricalresistance and low thermal conductivity in an axial directionperpendicular to the plane of the sheet as indicated by the axis 2 inFIG. I. In the plane of the sheet, X-Y, i.e., the basal plane,perpendicular to and radially of axis 2 the material exhibits thecharacter of having relatively high thermal conductivity and lowelectrical resistance though the latter characteristic is not ofparticular significance to the present invention.

Properties of a typical grade of pyrolytic carbon suited to theinvention are as follows:

at [000C at 30C. X-Y Thennal same Conductivity 2 Joule cm/cm* sec C 2"Thermal I same Conductivity .02 .loule cm/cm sec C "X-Y Electrical225x10 ohm cm 400X 10 ohm cm Conductivity "2" Electrical .35 ohm cm .6ohm cm In order to achieve a self-supporting tube with the desiredanisotropic orientation, a plurality of cylinders 11 of the material arecut out of the sheet 10 and cylinders 11 are each individually providedwith a precision bore 12 which in as sembly, provides the desiredoptical aperture. At opposite ends of each cylinder suitable malethreads 13 and female threads 14 are provided and which enable thecylinders 11 to be assembled into a self-supporting tubular form withsolid electrical contact as if the form had been made into an integralstructure both electrically and physically. The basic tube assembly iscompleted by the addition of the mounting pieces 15, 16 which asindicated are provided with mating bores l7, l8 and are preferablytapered as indicated at 19 to facilitate and confine the heat flowduring laser operation. While mounting pieces l5, 16 might be formed ofa material such as pressed graphite or a suitable metal, it ispreferable that the same anisotropic material, namely, the pyrolyticgraphite, be employed.

It may be noted that while screw connections are indicated as a means ofjoining cylinders 1 l and mounting pieces l5, 16 other means such as asuitable carbon glue might be employed. However, any securing meansemployed should provide a solid, continuous, electrical contact betweenthe cylinders 11 and should adapt to the extremely high temperaturesencountered and not lead to differential expansion problems. Further,the tube in use should preferably be self-supporting and maintain theaccurate alignment of the axial bore of the tube. Of particular interestto the present invention is the fact that the method of manufactureillustrated by FIG. 1 allows a self-supporting anisotropic tube havinghigh radial heat conductivity and an axial semi-conductor electricalcharacter to be obtained from sheet material whose conversion to thedesired tubular form and anisotropic orientation is otherwise notreadily apparent. While the self-supporting feature is desirable it willbe understood that certain embodiments such as the water-cooledembodiment next described may lend themselves to utilizing external orother support means for the tube.

Reference is next made to FIGS. 2 and 3 which illustrate a simplifiedsomewhat schematic air cooled laser system embodying the invention andin which only those components necessary to an understanding of theinvention are illustrated. For example, the details of the powersupplies, solenoid, mirrors, mirror adjustments and the like are notshown since the same may be conventional and are well known to thoseskilled in the art. Further, it will be apparent that the discharge in agas laser of the type found in the invention may be initiated in variousways such as by a Tesla coil and while the invention in one aspect isconcerned with the manner in which the discharge is initiated and thensustained once it has been initiated, no attempt is made in the drawingsor description to deal with any specific auxiliary circuitry forinitiating the discharge such as would normally be required in the laserembodiments illustrated in the drawings.

Again referring more specifically to FIGS. 2 and 3, a suitable gashousing 20 is formed of a suitable glass material such as quartz, Pyrexor the like so as to provide an enclosure generally indicated at 21 forenclosing the active discharge region, an anode region enclosure 22, acathode region enclosure 24 and the usual gas return path enclosure 25.Brewster windows 30, 31 are mounted in the usual manner and along thesame axis are mounted the required adjustable reflecting mirrorassemblies generally indicated at 32, 33. The cathode 35 and the primaryanode 36 may be of conventional construction and are connected tosuitable power supplies not shown. While shown in a location remote fromthe tube, it should be understood that the anode 36 and cathode 35 couldbe located in other positions known to the art. As later explained itshould also be understood that in one operating mode the necessity foranode 36 is eliminated. Starting or initiation of the discharge may beeffected in various ways as previously stated and to enhance the laseraction there is shown the usual magnetic coil 37 which should beunderstood to be mounted so as to surround the enclosure portion 21constituting the active region.

Referring to FIG. 3 the tube assembly composed of the assembledcylinders 11 and the mounting pieces 15, 16 is shown mounted in theenclosure 21 forming part of the gas housing 21. Mounting pieces 15, 16are preferably snugly fitted and it will be noted that an electricallyconducting cylinder 40 is mounted in the end of mounting piece 15.Cylinder 40 is preferably formed of a refractory, electricallyconductive material, such as tantalum, and is press-fitted into mountingpiece 15 to provide a means for electrically connecting a secondaryanode 41 through connecting wire 42 to the mounting piece 15. Theutilization of secondary anode 41 is discussed later and it may be notedthat secondary anode 41 is assumed to be connected to a suitable powersupply not shown but the character of which will be indicated by thedescription to follow. It should also be noted that the gas housing 20is filled with a suitable gas of a type which will assume an ionic stateat the discharge temperature. Gases suited to the invention includeargon, krypton and xenon though this list is given merely by way ofexample and not by way of limitation.

Prior to dealing with any particular mode of operating procedurereference will be made to FIGS. 4 and 5 which represent a laser systemcomparable to the system of FIGS. 2 and 3 except that the system ofFIGS. 4 and 5 is designed for water-cooled rather than air-cooledoperation. To the extent that the two systems use the same componentslike numerals designating the parts have been used. In particular, itwill be noted that the water-cooled system uses an enclosure designatedby 21' and forming part of a gas housing 20 which includes an inlet 50and an outlet 51 for connection to a suitable source of cooling waternot shown. The tube itself is enclosed in a further housing 23 whichsurrounds the tube and the mounting pieces l5, 16. Housing 23 is sealedto housing 21' or provided with gaskets to contain the cooling liquid.Both housing 21' and housing 23 may be formed of quartz and suitablysealed. The volume of flow and the inlet and outlet temperatures will ofcourse vary with the application but generally it may be stated thatcooling water at normal tap temperature can be adapted to cool lasersystems of the powers envisioned within the scope of the invention.

A typical starting procedure and mode of laser operation will next bedescribed. The gas housing is first pumped down to within the range ofmillimeters mercury pressure of air and is filled with a suitable gassuch as argon at approximately one-half millimeter pressure. Themagnetic solenoid '37 is energized to produce a magnetic field strengthin the tube in the range of 500 gauss and which acts to enhance thelaser action as known to the art. A suitable potential which may, forexample, be 500 volts DC. is applied between the cathode 35 and thesecondary anode 41. Since the tube of the invention is a "conductor itacts to shield any attempt to fire the gas in the tube itself. At thisstage it is therefore necessary to break the gas down to create astarting discharge between the cathode 35 and the cathode end of thetube and one means of doing this is to excite the gas by use of anauxiliary Tesla coil or by means of a pulse of suitable intensity suchthat a low starting current are is created between the cathode and theend of the laser tube adjacent the cathode. Next, it is necessary toincrease the voltage or current by any suitable means such that thedischarge will be caused to move along the laser tube towards the anodeend of the tube and so as to terminate on the secondary anode 41. Inthis mode the discharge will follow the gaseous path within and definedby the laser tube. That the discharge will follow this mode of operationcan be explained by the fact that the voltage drop per unit length ofdischarge in the gas moving from the cathode to the anode end of thetube will be lower than the voltage drop per the same unit lengths oflaser tube. Expressed in terms of electrical im pedance it can be saidthe plasma offers a lower impedance. With the discharge extendingbetween the cathode 35 and the secondary anode 41, the laser can be saidto be in an operating laser mode.

To achieve an alternate operating mode, the operating procedure is totransfer the discharge from the secondary anode 41 to the primary anode36 and this is accomplished by applying a suitable voltage to theprimary anode 36 to effect the transfer and by using suitable externalmeans to break down the gas and establish a discharge between the anode36 and the anode end of the tube. However, it is important to note thatin this alternate mode when making this final transfer enough voltage isleft on the secondary anode 41 to maintain a uniform potential gradientalong the length of the laser tube the reason for this being laterexplained. It may also again be noted that transfer to the primary anode36 is optional and represents an alternate mode of operation to theextent that an operating laser mode may be maintained by utilizing thesecondary anode 41 and the tube itself as the only source of anodepotential during laser operation.

The anisotropic and semi-conductor character of the laser tubeconstruction offers a number of advantages. From a thermal viewpoint, itbecomes possible to mount such a laser tube in materials and forms ofmountings which do not require adaptability to extremely hightemperatures since the very character of the laser tube of the inventionsubstantially eliminates the usual extremely high level axial heatconduction. That is, whatever means are used to grip the ends of thetube remain essentially cool because of the nominal axial heatconduction. Furthermore, because of the excellent radial heat conductingproperties the heat which is primarily developed in the active dischargeregion can be dissipated radiallyand confined to that region. Coolingfluids, heat conducting fins and the like can thus be concentrated inthe area around the active discharge region which offers a materialadvantage from the viewpoint of engineering the heat transfer aspects ofany particular laser system. The overall thermal advantages thus reducethermal heating and the usual deterioration of the laser bore due tothermal heating.

A further advantage of the invention is that the semi-com ductorelectrical properties of the laser tube of the invention offeradvantages in the nature of the starting and operating techniques whichmay be employed. A particular advantage of a semi-conductor materialsuch as pyrolytic graphite is its ability to adapt to laser dischargewithout the usual deterioration of the laser bore caused by ionbombardment. That is, the laser tube of the invention offers a reductionin deterioration of the laser bore both because of a reduction of thethermal heating problem as well as a reduction of the ion bombardmentproblem. With regard to the latter, ion bombardment is known to bedestructive because of the high mass of the ions. Electrons, on theother hand, when impinging on a laser tube material with the same energyas ions will not effect comparable damage. Considering a laser tube boreas a discharge wall, it can be said that ion bombardment is caused bythe discharge wall having a potential more negative than that of theadjacent plasma column. The use of dielectric materials in laser tubescauses such a negative potential from an electron buildup on thedischarge wall. Furthermore, where the laser tube comprises a series ofconducting discs, the referred to stacked graphite carbon disc lasertube being an example, a damaging negative potential of the kindreferred to is produced since each disc adjusts to the average potentialof the plasma column at the point of contact to the disc. Thus, theanode side of each such disc is more negative than the plasma and ionbombardment is induced.

According to the invention, an electrical semi-conductor such as thementioned pyrolytic graphite material is employed and in a physical andelectrical configuration which allows the potential of the dischargewall to be maintained at the same potential as the plasma column at eachpoint of contact. This is accomplished in the invention by placing thepotential on the secondary anode 41 at or near the operating potentialof the primary anode 36. With this arrangement, the electron currentfrom the plasma to the discharge wall maintains all points of thedischarge wall at or near the same potential or positive with respect tothe potential of the column at the point of contact to the plasma.Furthermore, the discharge current itself is maintained primarily in thegaseous path and not in the tube material since the plasma column as anelectrical path forms a path of lower resistance than does theelectrical path provided by the material forming the laser tube itself.

While the laser tube construction shown in the preferred embodiment isessentially a cylindrical tube it should be noted that other physicalconstructions are encompassed within the scope of the invention. It isrecognized, however, that whether the tube is made in a true tubular,columnar or other geometrical form it must provide an optical aperture"and it must be of such form as to act as a guide" for the plasma columnin the sense heretofore explained. Within these boundary conditions,however, it can be seen that laser tube forms other than the simplecylindrical, tubular form described may utilize the teachings of theinvention and this will next be discussed.

It has already been mentioned that the laser of the invention leads to areduction in ionic bombardment and thus to an increase in laser tubelife. FIGS. 6, 7 and 8 deal with alternate forms of the invention inrespect to equalizing the tube and plasma potential gradients for thispurpose. To supplement the prior discussion of ion bombardment, it canbe shown that the drop through the plasma is non-uniform when in auniform cross-section bore, when under the influence of a uniform fieldand when in a laser mode, i.e., the voltage drop per unit length underthese conditions will be different from unit to unit along the tubelength. However, the potential, whatever its value, at the anode end ofthe tube will tend to be the same in both the gas and tube. Furthermore,it can be said that the potential at the cathode end of the tube,whatever its particular value, will tend to be equal in both the gas andtube. The tube of the invention, unlike the gas, when made of uniformcross section does exhibit a uniform drop per unit length. Therefore, ata particular point along the bore, the tube may tend to go negative withrespect to the gas at the same point and induce ion bombardment at thatpoint. To further reduce the possibility of ion bombardment at any pointof contact between the tube bore and the gas in the discharge region theinvention contemplates in alternate embodiments the employment of eitherelectrical or physical configurations as illustrated in FIGS. 6, 7 and 8to compensate for any tendency of the tube to go negative with respectto the gas. The tube of the invention because of its unique character ofbeing essentially an integral conductor" and at anodic potential isuniquely suited to these embodiments.

At least three practical means are available for the desired potentialgradient compensation. These include the concept of increasing the tubematerial cross section in the anode direction as illustrated by themodified tube 60 and in which the bore 61 is of uniform diameter. Itwill, of course, be understood that FIG. 6 is not intended to be drawnto scale and that any specific tube would have its particularcrosssection gradient adjusted to the electrical potential gradientrequired by the particular gas and other parameters of the system suchthat the walls of the bore 61 at all points remain positive or neutralwith respect to the gas.

Another means for achieving potential gradient adjustment is illustratedby FIG. 7 in which the modified tube 70 is provided with a bore 71 whichtapers toward the anode end so as to reduce the discharge path in theanode direction and thereby achieve the desired potential gradient inthe gas.

A further electrical means is schematically illustrated by FIG. 8 inwhich the laser system includes means for decreasing the solenoidmagnetic field in the anode direction and which acts to increase thepotential drop per unit length in the plasma. That is, it is known thatpotential drop in the plasma is directly related to the magnetic fieldproduced by the solenoid and thus by decreasing the magnetic field inappropriate amounts in the anode direction the magnetic field itself canbe used as a means for establishing the desired potential gradient andcontrolling ion bombardment. While not shown in detail it will beunderstood to those skilled in the art that suitable means fordecreasing the field could, for example, be inherent in the solenoiddesign or could be provided by compensating windings productive ofcontrolled opposing field conditions.

It is also contemplated that other semi-conductor materials having aresistance in the semi-conductor range of 10 to 10' ohm centimeters atthe laser operating temperature will be found useful in the anti-ionicbombardment and potential arrangement taught by the invention andirrespective of whether such materials exhibit the same anisotropiccharacter as does the pyrolytic graphite material used as an example inthe embodiment herein disclosed.

Having described the invention, what is claimed is:

1. A laser plasma guide comprising an elongated material having a boretherethrough for forming an optical path and a plasma containing wall,said material being a semi-conductor in the direction of elongation atthe operating temperature of said plasma and providing an electricalcontact over the total length of said bore.

2. A guide as claimed in claim I wherein said material is anisotropicand is highly thermally conductive in the direction radially of saidmaterial direction of elongation.

3. A guide as claimed in claim 2 wherein said material is pyrolyticcarbon.

4. A guide as claimed in claim 1 wherein at the operating temperature inthe active region of said guide the resistance of said material is inthe range of 10 to 10 ohm centimeters.

5. A guide as claimed in claim 3 wherein said guide in the active regionis formed as a substantially cylindrical tube.

6. A guide as claimed in claim 3 wherein said tube is formed ofindividually secured cylinders in which said bore is a common axial boreof said cylinders arranged to provide said aperture and collectivelyform said guide.

7. A guide as claimed in claim I wherein said bore is tapered inwardlytoward one end of said guide and in relation to a selected potentialgradient established within said bore.

8. A guide as claimed in claim 1 wherein the diameter of said bore isuniform and the cross section of said guide increases toward one end ofsaid guide and in relation to a selected potential gradient establishedwithin said bore.

1. A laser plasma guide comprising an elongated material having a boretherethrough for forming an optical path and a plasma containing wall,said material being a semi-conductor in the direction of elongation atthe operating temperature of said plasma and providing an electricalcontact over the total length of said bore.
 2. A guide as claimed inclaim 1 wHerein said material is anisotropic and is highly thermallyconductive in the direction radially of said material direction ofelongation.
 3. A guide as claimed in claim 2 wherein said material ispyrolytic carbon.
 4. A guide as claimed in claim 1 wherein at theoperating temperature in the active region of said guide the resistanceof said material is in the range of 10 3 to 106 ohm centimeters.
 5. Aguide as claimed in claim 3 wherein said guide in the active region isformed as a substantially cylindrical tube.
 6. A guide as claimed inclaim 3 wherein said tube is formed of individually secured cylinders inwhich said bore is a common axial bore of said cylinders arranged toprovide said aperture and collectively form said guide.
 7. A guide asclaimed in claim 1 wherein said bore is tapered inwardly toward one endof said guide and in relation to a selected potential gradientestablished within said bore.
 8. A guide as claimed in claim 1 whereinthe diameter of said bore is uniform and the cross section of said guideincreases toward one end of said guide and in relation to a selectedpotential gradient established within said bore.